Uninterruptible power supply (UPS) systems are commonly used in installations such as data centers, medical centers and industrial facilities. UPS systems may be used in such installations to provide backup power to maintain operation of computer, medical devices and other critical equipment in event of failure of a primary utility supply. These UPS systems commonly have an “on-line” configuration including a rectifier and inverter coupled by a DC link that is also coupled to a backup power source, such as a battery. Other UPS configurations may also be used, such as standby and line-interactive configurations.
UPS systems may be necessary in situations where power fluctuations or outages frequently occur because they can provide backup power circuitry that keeps vital systems operational in case the power supply is cut off. Under conditions involving short fluctuations or voltage disruption, the UPS can maintain constant power to keep loads running, and if electrical failure occurs, it can activate reserve power to keep systems operating until they can be safely shut down. In addition, these UPS systems can often also reduce the risk posed by harmonic disruptions and line transients. Thus, an effective UPS usually includes several of the following features: regulated output voltage with low harmonic distortion unaffected by input voltage or load changes; an input current with reduced harmonic distortion; a low degree of electromagnetic interference and acoustic noise; minimal transition times between normal and backup operations; high levels of reliability and efficiency; and relatively low cost, weight, and size requirements
Thus, put simply, a UPS is a device that provides backup power when utility power fails, either long enough for critical equipment to shut down gracefully so that no data is lost, or long enough to keep required loads operational until a generator comes online; and conditions incoming power so that all-too-common sags and surges don't damage sensitive electronic gear.